1. Field of the Invention
The present invention relates to gratings used to generate electrical fields in an ion flight path within a mass spectrometer.
2. Description of the Background
Time-of-flight mass spectrometers (TOFMS) are widely used to analyze molecular species, especially larger biomolecules. In such instruments molecules are ionized and the resulting ions are separated by their total flight time through electrical fields located between an ion pulser and a detector. The total flight time depends on the mass-to-charge ratio of each of the ions separated, and thus the mass of the ionized molecules can be determined.
The total flight time is also a complicated function of both the ion energy and the potential distribution of the electrical fields through which the ions travel. Thus, to achieve high resolution of ions having different mass-to-charge ratios, both the ion energy and the potential distribution of the electrical fields must be precisely determined and controlled. A small distortion in the electrical fields usually results in a significant distortion in the flight time, which reduces mass resolution.
Within a TOFMS, electrically conducting mesh screens, such as screens 100 and 150 illustrated in FIGS. 1A and 1B, respectively, are commonly placed between the ion pulser and the detector and used to generate and separate electrical fields of different strengths. The mesh screens are also used to improve the homogeneity of the electrical fields through which the ions travel. A problem with the screens, however, is they can reduce the sensitivity of the mass spectrometer. The screens are typically square or rectangular grids of horizontal 110 and vertical 115 wires, as shown on screens 100 and 150 in FIGS. 1A and 1B. Such screens usually have an optical transparency, which correlates to the transparency of the screen to ions, of 60% to 90%. For example, a commonly used mesh screen, part no. MN-23, supplied by Buckbee Mears, St. Paul, Minn., has an optical transparency of 85%. Therefore, many ions traveling through screen 100, 150 will strike the wires 110 and 115, and not make it through the screen to the detector.
Furthermore, in a typical TOFMS analyzer ions may pass through up to eight such mesh screens. Conventionally, the arrangement of the grid wires of these screens with respect to each other is arbitrary, i.e., neither horizontal nor vertical grids of the adjacent screens are intentionally aligned. FIG. 2 illustrates an ion packet 202 passing through two screens 205 and 207. As shown, some of the ions 210 passing the grid wires 110 of a first screen 205 may strike the grid wires 110 of an adjacent screen 207, resulting in ion transmission loss. An ion packet that passes through eight mesh screens in the flight path may have a total transmission loss of more than 73%, i.e., only 27% of ions in an ion packet generated at the ion pulser is detected at the detector. As more screens are added between the ion pulser and the detector, the transmission loss increases and the sensitivity of the instrument is reduced.
The mesh screens may also reduce sensitivity of the instrument by causing background noise in a spectrum. Because some of the ions strike the grid wires 110, 115 of the screens, unwanted particles such as secondary electrons, secondary ions, neutral particles, or stray ions will be produced. Depending on the location in which these electrons and ions are generated, these unwanted particles can arrive at the detector and be detected as noise.
In addition to reducing sensitivity, the grids may also cause time distortion of the ion packets, which degrades the mass resolution. The field near the grid wires can deflect ions, which produces a distortion in the flight time of the ions. Additionally, if the grids are not flat, but bent or uneven, the field is not completely homogeneous, which also causes a distortion in the ion flight time. For example, in a TOFMS instrument in which a 5 kV ion acceleration is applied, a non-flatness in a grid of xc2x110 xcexcm over the cross-section of the ion beam (typically between 20 mm to 50 mm wide) can cause a 2 nanosecond error in the flight time for an ion of mass 10,000 amu. Such a 2 nanosecond error can be significant. For example, if the error due to non-flatness is excluded, a 10,000 amu ion having a total flight time of 100 xcexcs may typically have an error of 5 nanosecond due to other error sources, such as imperfect energy focusing. In this case the mass resolution is 10,000 (i.e., 100 xcexcs/(2xc3x975 ns)). When a 2 nanosecond error due to imperfect flatness of the grid is added to the other sources of error (2 ns+5 ns), the mass resolution drops to 7,140 (100 xcexcs/(2xc3x977 ns)), a 28.6% reduction in mass resolution. Because the grid screen is normally very thin ( less than 5 xcexcm), it may be stretched to obtain some degree of flatness, and the screen may be stretched in both the horizontal and vertical directions. However, any uneven stretching in one direction can cause significant deformation in the grids, and thus it is extremely difficult to achieve a high degree of flatness.
A method and apparatus for generating electrical fields within the ion flight path of a mass spectrometer are provided. The method and apparatus advantageously provide high transmission efficiency of ions, thus increasing the sensitivity of the mass spectrometer. The method and apparatus also reduce distortions in ion flight times, thus improving mass resolution of the ions.
In one embodiment, gratings formed from a planar array of parallel conductive strands and electrically connected to a voltage source are used to generate electrical fields within an ion flight path of a mass spectrometer. The gratings are placed in the ion flight path so that at least a portion of the conductive strands traverses the flight path. The gratings do not have any conductive strands that are perpendicular to the parallel conductive strands and that also traverse the ion flight path.
The gratings may be arranged within the ion flight path so that the conductive strands of a second grating are aligned behind the conductive strands of a first grating, with respect to the ion flight path. This allows the majority of ions that pass through the first grating to pass through the second grating.
The spacing between conductive strands may be different in each of the gratings within the ion flight path. In one example, the spacing between conductive strands of each of the gratings within the ion flight path is an integral multiple of the spacing between the conductive strands of the grating that has the smallest spacing between conductive strands.
The gratings may be mounted on frames to position the conductive strands within the flight path. One of the ends of the parallel conductive strands may be electrically connected to a conductive support strip and the other ends connected to a support strip that is not necessarily conductive. The support strips may include a plurality of precisely positioned holes and each frame may include a plurality of corresponding holes. The holes on the conductive strip and frame allow the gratings to be aligned and mounted onto the frames, using fasteners such as screws.
The frames may also be used to stretch the gratings, pulling both ends of each of the conductive strands outward from the array and away from each other, to flatten the gratings.